The present invention relates to a steam turbine inlet for providing substantially uniform mass flow and velocity as the steam flows axially into the first stage(s) and particularly relates to a steam inlet having a linearly varying cross-sectional area in a circumferential direction from inlet ports adjacent the horizontal midline to upper and lower vertical centerlines of the fixed casing whereby losses due to non-uniform flow are minimized or eliminated. The present invention also relates to a method of retrofitting existing steam turbines to provide a uniform mass flow and velocity in the inlet to the first stage nozzles.
In steam turbines, for example, low pressure steam turbines, feed steam from a high pressure section flows into a low pressure steam inlet, typically including a pair of inlet ports generally on opposite sides of the turbine housing and an annulus. The steam flow through each steam inlet port splits in opposite circumferential directions for flow through arcuate sections of the annulus, which typically have a constant cross-sectional area. As the flow follows the circumferential path of the inlet annulus, the steam feeds radially inwardly and turns axially into the first stage nozzles. In split flow axial steam turbines, the radial inward flow from the annulus splits for flow in opposite axial directions to the first stage nozzles.
Ideally, the low pressure inlet turns the steam 90xc2x0 into axial flows with minimum loss. However, with an annulus of constant cross-sectional area within the housing in communication with steam inlet ports, considerable energy losses occur due to a decrease in steam velocity as it traverses the circumferential extent of the annulus in directions away from the inlet ports. With a substantially constant cross-sectional flow area about the annulus, the mass flow is not constant and a non-uniform velocity profile at the axial inlet(s) to the first stage nozzles occurs. Accordingly, there is a need for an improved steam inlet for a steam turbine wherein the steam flow will maintain uniformity throughout the inlet, thereby eliminating losses due to non-uniform flow and affording a substantially uniform velocity profile as the steam enters the first stage nozzles.
In accordance with a preferred embodiment of the present invention, there is provided a steam inlet configured to provide a uniform mass flow of steam at substantially uniform velocity in radial inward and axial directions for delivery to the first stage nozzles. To achieve this relatively constant mass flow and uniform velocity profile, the inlet includes an annular casing defining a chamber of substantially progressively reduced cross-sectional area in a generally circumferential direction away from the steam inlet ports. By progressively decreasing the cross-sectional area, mass flow and uniform velocity are substantially achieved.
Particularly, in a preferred embodiment of the present invention, there is provided a split flow axial steam turbine having a casing defined by outer peripheral and side walls in communication with steam inlet ports generally along opposite sides of the turbine housing adjacent the horizontal midline. The steam flow through the inlet ports splits for flow along upper and lower portions of the chamber defined by the casing. The cross-sectional area of the chamber decreases in a direction away from each inlet port to a minimum cross-section at locations substantially medially between the steam inlet ports along opposite circumferential steam flow paths in upper and lower housings containing portions of the chamber. The casing thus generally provides quadrants of steam flow passages of progressively reduced cross-sectional areas from the inlet ports to minimum cross-sectional areas approximately 90xc2x0 away from the inlet ports. By progressively reducing the cross-sectional area, the mass flow and velocity remain substantially uniform in radial inward and axial directions, thereby reducing energy losses.
The steam inlet casing may be provided as part of original equipment manufacture or may be provided as a retrofit to existing steam turbine inlets. In the latter case, the annulus defined by the original steam turbine housing may be provided with one or more arcuate unitary casings having outer peripheral and side walls defining the progressively reduced cross-sectional flow passage about the rotor. The casings can be preformed, for example, for installation in each quadrant, or the walls of the casings can be fabricated and secured individually to the turbine housing to define flow passages of progressively decreasing cross-sectional area in a direction away from the steam inlet ports.
In a preferred embodiment according to the present invention, there is provided in a steam turbine, a steam inlet comprising a generally annular casing having an outer surrounding peripheral wall and a pair of axially spaced side walls extending inwardly to define a generally annular chamber within the casing and at least one generally annular steam outlet generally centrally of the casing in communication with the chamber for flowing steam axially outwardly through the outlet into the first stage of the turbine, a pair of steam inlet ports spaced from one another about the casing for receiving steam and transmitting steam into the chamber, the chamber having a substantially progressive reduction in cross-sectional area in a generally circumferential direction away from the steam inlet ports to provide a substantially uniform flow of steam about the chamber in a generally radially inward direction.
In a further preferred embodiment according to the present invention, there is provided in a split flow axial steam turbine, a steam inlet comprising a generally annular casing having an outer surrounding peripheral wall and a pair of axially spaced side walls extending inwardly from the outer wall to define a generally annular chamber within the housing, a pair of steam inlet ports spaced from one another about the casing for receiving steam and flowing the received steam into the chamber, a pair of axially spaced, generally annular steam outlets in communication with the chamber for flowing steam in opposite axial directions through the outlets to stages of the turbine, the chamber having a progressive reduction in cross-sectional area in a generally circumferential direction away from the steam inlet ports to provide a generally uniform flow of steam from the chamber through and about the steam outlets.
In a further preferred embodiment according to the present invention, there is provided in a split flow axial steam turbine having a housing with an annulus for receiving steam from a pair of circumferentially spaced steam inlet ports and a pair of axially spaced steam outlets radially inwardly of the annulus for receiving steam from the annulus for flow in opposite axial directions to stages of the turbine, a retrofit steam chamber for the annulus, comprising a plurality of generally arcuate casings each having an outer peripheral wall and a pair of axially spaced side walls extending inwardly from the outer wall to define a generally arcuate passage, the arcuate casings being disposed within the annulus in communication with the steam inlet ports, respectively, each of the arcuate passages having a progressive reduction in cross-sectional area in a generally circumferential direction away from the steam inlet ports to provide a generally uniform flow of steam from the chamber through and about the steam outlets.
In a further preferred embodiment according to the present invention, there is provided in a split flow axial steam turbine having a housing with an annulus for receiving steam from a pair of circumferentially spaced steam inlet ports and a pair of axially spaced steam outlets radially inwardly of the annulus for receiving steam from the annulus for flow in opposite axial directions to stages of the turbine, a method of retrofitting a steam inlet to obtain a generally uniform velocity of steam flowing axially through and about the steam outlets, comprising the steps of forming a plurality of arcuate casings each having an outer peripheral wall and a pair of axially spaced side walls extending inwardly from the outer wall to define a generally arcuate steam flow passage of decreasing cross-sectional area from one end to an opposite end, installing the casings as unitary casings or as discrete peripheral walls and side walls in the annulus of the housing with larger cross-sectional ends thereof in communication with the inlet ports and with passages in communication with the axial steam outlets for flowing steam at substantially uniform velocity about the outlets in opposite axial directions.